Giant magnetoresistive (GMR) sensors are employed in read heads of magnetic data storage devices to read data recorded on a recording medium, such as a rotating disk. The data are recorded as magnetic domains in the recording medium. As the data moves past the head, the data causes changes in magnetic flux to the head. These changes in the magnetic flux in the head causes changes in the electrical impedance of the GMR sensor, which are detected by applying a bias or sense current through the sensor and detecting changes in the voltage drop across the sensor. As a result, the changing voltage across the sensor is representative of the data recorded on the recording medium.
Typical magnetic sensors utilizing the GMR effect, frequently referred to as “spin valve” sensors, are known in the art. A spin valve sensor is typically a sandwiched structure consisting of two ferromagnetic layers separated by a thin non-ferromagnetic layer. One of the ferromagnetic layers is called the “pinned layer” because it is magnetically pinned or oriented in a fixed and unchanging direction by an adjacent anti-ferromagnetic layer, commonly referred to as the “pinning layer” through an anti-ferromagnetic exchange coupling. The other ferromagnetic layer is called the “free” or “unpinned” layer because the magnetization is allowed to rotate in response to the presence of external magnetic fields. The resistivity of the stack varies as a function of the angle between the magnetization of the free or active layer and the magnetization of the pinned layer. Contact layers are attached to the GMR sensor to supply the sense current and permit measurements of the voltage drop across the sensor or its resistance.
Areal data density for a magnetic recording media is the product of the bit density along the length of the recording tracks and the density of those tracks in a direction normal to the track length. As track density increases, track width and spacing decreases, and the areal density increases. The smaller track widths and spacing require read heads with more narrow widths and the ability to prevent side-reading. Side-reading occurs when a magnetic head responds to changing magnetic fields outside the bounds (width) of the head. This side-reading is a source of noise in the recovered data signal, and a source of cross-talk, a phenomenon where the read-head reads data from two or more adjacent tracks. Consequently, the effects of side-reading in a read head is a limiting factor on the spacing between adjacent tracks, and hence a limiting factor to increased areal density.
As the width of the read head becomes smaller to allow for higher recording track densities, demagnetization fields can have a substantial effect on the operation and sensitivity of the GMR sensor. The demagnetization fields are produced by the pinned and free layers due to their magnetic fields which traverse the thin non-ferromagnetic layer that separates them. These demagnetization fields typically have the effect of reducing the change in the relative angle between the magnetization of the pinned and free layers for a given applied magnetic field from the recording medium. As a result, the sensitivity of the GMR sensor is reduced. Additionally, the demagnetization fields can push the biased point in the reader transfer curve of the GMR sensor to its non-linear region creating symmetry and stability problems. For many spin valve sensor designs, the demagnetization fields are a limiting factor to the minimum width of the sensor and thus a limiting factor to the areal density of the data they can reliably read.
Another factor used to determine the maximum areal density of data a spin valve sensor is capable of reading is the length of a transducing read gap (in the direction of track length) of the spin valve sensor. The spin valve sensor is designed such that the length of the transducing read gap is not more than the length of a single flux element, corresponding to a bit of data, on the track. Successive flux elements are recorded in opposite magnetic orientation along the track length, so that by limiting the length of the transducing gap to that of a single flux element, two or more successive flux elements are not read simultaneously, which could lead to their cancellation. As a result, it is desirable to minimize the transducing read gap or the shield-to-shield spacing between the bottom and top shields of the spin valve sensor in order to maximize the areal density of data that the spin valve sensor is capable of reading.
Some spin valve sensors employ permanent magnets abutting opposite sides of the magnetoresistive elements of the sensor stack. These heads are generally referred to as “abutted-junction” magnetoresistive and GMR heads. As mentioned above, usually the head is formed by forming the magnetoresistive elements or sensor stack on a planar bottom shield and thereafter forming the permanent magnet and contact layers. The height configurations of the permanent magnet and contact layers often require that the top shield, positioned opposite the bottom shield, be of varying distance from the bottom shield. More particularly, the thickness of the permanent magnet in contact layers together are often greater than the thickness of the sensor stack. As a result, the spacing between the top and bottom shields increases (along the track length) outside the width of the sensor stack. This increase in spacing between the top and bottom shield can cause the spin valve to read multiple flux elements thereby resulting in a canceling effect on the read signal.
There exists a continuing demand for increased areal densities in magnetic data storage systems. To accommodate this demand, advancements in GMR sensor design are required in the areas of reducing the negative effects of demagnetization fields and improving shield-to-shield spacing.